This invention relates generally to an apparatus for inducing vibrations in a plate type structure and, more particularly, to an airfoil blade, such as is used in various components of a gas turbine engine.
A prime cause of airfoil blade failures occuring during testing and development programs has been, and remains, the response of the airfoil blade to chordwise bending modes resulting from gaseous flow impingement during rotor blade/stator blade passing excitation. These "striped-mode" responses are characterized by nodal patterns on the airfoil blade resulting in relatively large panel sections, continuous in the span-wise direction, vibrating at frequencies higher than the primary airfoil blade excitation modes.
In addition, recent engine test experience has taught that second order of a blade or vane passing integer order of excitation of these modes is of aero-mechanical interest.
Further, mechanical developments of airfoil blades will be significantly more straightforward if vibratory fatigue tests to crack inception can be conducted upon airfoil blades being shaken in these modes, many of which are characteristically difficult to drive at high strain levels with prior art vibration-inducing test apparatus.
Therefore, what is needed in the art and is not presently available is apparatus capable of exciting such higher order modes to a strain level in an airfoil blade (or a plate type structure) sufficient to induce fatigue failure and to allow meaningful and repeatable vibratory strain distributions to be observed, recorded, studied, and the like.